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Back in early 1980’s, I lived and breathed the world of the Apple II, Atari 800, Commodore 64, and their brethren. I could PEEK and POKE those machines like nobody’s business, and I spent countless hours writing programs, playing games, or just fiddling around. In contrast to today’s PCs, the computers of that era were inviting to tinkerers, with a comparatively simple hardware design and a BASIC prompt at boot-up.

As a computer engineering major in college, I learned the details of digital logic design. I even built a rudimentary computer on a prototyping kit built into a suitcase: MIT’s infamous “Nerd Kit”. But at the end of the semester, it was all torn down, I went on to a career in software, and that was that.

More recently, I learned of various projects to build simple computers similar to those 80’s machines, constructed entirely of discrete logic chips like counters, adders, flip-flops, and NOR gates. No Pentiums or PowerPCs here– these people built their own CPUs from the ground up, along with the memory subsystem, I/O, and everything else the computer required. I had stumbled onto the world of the homebrew CPU. To create such a computer required a detailed microarchitectural design, custom instruction set design, custom software tools like assemblers and compilers, and of course a custom circuit board or three populated with lots of fat DIP chips and a big mess o’ wires. Projects like the Magic-1, D16/M, and Mark 1 FORTH Computer showed me the way.

I decided to build a homebrew CPU computer of my own. It was a big mess o’ wires.

Construction is documented in 100 blog posts, ranging from the original idea through dozens of setbacks to a final demo for thousands of people at the 2009 Maker Faire.

The Hardware

Big Mess o’ Wires 1 is an original CPU design. It does not use any commercial CPU, but instead has a custom CPU constructed from dozens of simple logic chips. Around this foundation is built a full computer with support for a keyboard, sound, video, and external peripherals.

Initial design began in November 2007 with a high-level sketch of the CPU internal design. A simplified Verilog hardware simulation proved the key design details. Construction began in earnest in February 2008, using a large wire-wrap board to interconnect the 50 or so chips needed. In April, a half-finished BMOW 1 booted up for the first time, computing fibonacci(12) = 144 using a simple ROM-based program. One by one the original system goals and stretch goals were met, including VGA video, three voice audio, BASIC, and a bootloader for communication with an attached PC. BMOW 1 eventually gained the ability to run complex programs written in assembly or compiled from C. The main construction phase ended in February 2009, with the completion of a customized case to house everything. As of March 2009, Big Mess o’ Wires 1 is fully functional, but will probably never be “finished”.

Architecture

BMOW 1 borrows liberally from other homebrew designs, as well as the MAYBE design presented in the book Computation Structures by Stephen Ward and Robert Halstead. Data busses are 8 bits wide, and the address bus is 24 bits. Four 8-bit registers are used for general data, and three 24-bit registers store the program counter, stack pointer, and a scratch/working address pointer. Registers and the arithmetic and logic unit are interconnected by one data bus, while RAM, ROM, and memory-mapped hardware devices use a second data bus. The ALU also has dedicated left and right data input busses.

Machine language instructions are implemented as a series of micro-instructions, stored in three parallel ROMs to create a 24-bit microcode word. One micro-instruction is executed each clock cycle, and the micro-instruction bits are used directly as enable and select inputs to control all the chips in the machine. Up to 16 micro-instructions may be needed to implement a single machine language instruction.

Note: Some additional devices are not shown here, including the VGA display circuitry and real-time clock.

24-bit addresses allow for up to 16MB of memory, but only a little more than 1MB of combined RAM and ROM is installed. The most-significant byte of the address is called the bank byte, and is normally invisible to programs. The standard instruction set presents a 16-bit interface to programs, with most instructions implicitly referencing the current bank. Cross-bank references are possible, but awkward (think x86 segment registers).

A 512K ROM contains a bootloader/menu program. A USB-to-TTL interface based on an FTDI chip provides an easy way to move data to and from a connected PC. A standard PC keyboard with PS/2 connector is used for keyboard input, and a 24×2 character text LCD serves as a debug output display. Custom video circuitry drives a standard VGA monitor, with a maximum resolution of 512 x 480. A three-voice programmable sound generator provides music and sounds.

BMOW 1 is built on an Augat wire-wrap board pre-populated with thousands of wire-wrap pins. The chips are pushed into the board without soldering, and can be easily removed, similar to a prototyping breadboard. Unlike a breadboard, the pins are individually connected on the underside of the board according to the needs of the circuit design. A wire-wrap tool is used to wrap stripped wire ends tightly around each pin. Wires can be removed fairly easily in case of a mistake. BMOW 1 contains about 2500 such wire wraps.

Specs

Current clock speed is 2MHz. It could theoretically go to about 3MHz (untested).

512 KBytes of RAM, 512 KBytes of ROM.

Power draw is 10 Watts, 2.0A at 5V.

VGA video output is 512×480 with two colors, or 128×240 with 256 colors.

Audio and music is provided by a three-voice programmable sound generator.

Layout – Shows the location of every chip on the BMOW 1 system board, color-coded by subsystem. More discussion of the layout is in this post.

Schematic Diagrams – The whole enchilada: detailed diagrams showing every chip, every pin, and every connection.

Wire List – A spreadsheet documenting every wire in the machine, its endpoints, and the order the wires were added. Essential for navigating the insane rat’s nest of wiring beneath the board. (The spreadsheet has embedded macros. Excel may warn you about this.)

Microcode – Source listing for the microcode that implements BMOW 1’s high-level machine instructions. Each high-level (programmer visible) instruction is implemented as a tiny program of 1 to 16 micro-instructions. The high-level instruction set that’s implemented in this microcode is a close cousin to 6502 assembly language.

Microcode Assembler – A PC-hosted tool that reads the microcode source listing, and generates the binary data to be programmed into the microcode ROMs. It’s a custom assembler for microcode.

PC Bootloader – A PC-hosted GUI tool for downloading a program to BMOW 1’s RAM and executing it.

Program Assembler – A PC-hosted command-line tool for assembling BMOW 1 source code into executable programs. It’s a customized version of the Acme 6502 assembler, with BMOW 1-specific extensions.

Software Examples – Source listings from BMOW 1 programs, including the bootloader client that runs on the BMOW 1 hardware, and audio/video demos.

Prototype Verilog Description – A Verilog model for an early prototype of the BMOW 1 CPU core. This is badly out of date now, and there are a couple of places where I cheated and wrote non-synthesizable code, so you couldn’t make hardware directly from this. Mainly for historical interest.

Modified Atari Font Map – The 128×128 pixel font map originally used for BMOW, taken from the Atari 8-bit computer. Each letter is 8×16, and there are 128 letters in the font map. The image shown here is actually 256×256 and must be reduced 50%. Also, the raw binary version of the font map, already reduced and with the PNG header stripped out.

8×16 Font Map – The 128×128 pixel font map used for the final version of BMOW, adapted from an X-Windows font. Each letter is 8×16, and there are 128 letters in the font map. The image shown here is actually 256×256 and must be reduced 50%. Also, the raw binary version of the font map, already reduced and with the PNG header stripped out.

BMOW 1 Photo Gallery (click images to see high-res versions)

Homebuilt CPUs WebRing

Join the ring?

David Brooks, designer of the Simplex-III homebrew computer, has founded the Homebuilt CPUs Web Ring.
To join, drop Dave a line, mentioning your page’s URL. It will then be added to the list. You will also need to copy this code fragment into your page.

121 Comments so far

Hello!
Wow!
Back during the days of the Apple and the R6502, I, myself happened to be a very good programmer. In both BASIC and assembler. So much so that I would be able to diagnose a problem for someone else from just by viewing a sick machine’s responses.

The song in the AY-3-8913 demo video is called Agent X II, by “The *genious* Tim Follin”. It’s a Spectrum ZX tune that I downloaded as part of a collection from http://bulba.untergrund.net/music_e.htm . No MP3 version, sorry.

WOW!!!
Just looking at it brings tears to my eyes.
You’ve completed one of my life long goals, I just hope I’ll be able to do something like that before I die…
You’ve got all my respect, and am Now an Internet Hero.

It’s been on and off since November 2007; some weeks I’m really busy and some weeks I don’t touch the computer at all. On average I’d guess I spend about 10 hours a week on it, which is a few weeknights and a Saturday afternoon.

[…] Big Mess o’ Wires » BMOW 1 A custom CPU constructed from dozens of simple logic chips. Around this foundation is built a full computer with support for a keyboard, sound, video, and external peripherals. (tags: diy hardware electronics) […]

envious May 30th, 2009
4:54 am

dude, this happens to be my -ultimate- goal with computer science. You are seriously right up there with a figurative god.

typo May 30th, 2009
9:56 am

Could you estimate how many transistors you used to compare em with intel cpu’s millions and billions?

[…] Big Mess o’ Wires – the home-made computer Wanna build your own computer (in you garage)? Maybe you can learn something from this guy: http://www.bigmessowires.com. He made an amazing work with his BMOW 1 (Big Mess o’ Wires). […]

Cool! Nice work. I wanted to design a CPU, having worked in circuit design with MSI logic & PALs back in the ’80’s. Do I presume correctly that 3-state data allows for multiplexing (selection of registers)?

Regarding the electrical interference with many wires: it is a problem, but not a fatal one. I tried to avoid running many wires parallel to each other, preferring to run them at an angle, to minimize the crosstalk. The system speed is also pretty slow (2 MHz), which means transient noise has more time to settle out before the next clock edge. The CPU still does crash occasionally, though, which I’m guessing it a noise problem.

olefowdie July 15th, 2009
7:28 am

I would love to build a “Big Mess O’ Wires” would you consider making a guide book to building one?

So, does this mean you could hand 1250 or so people each a length of wire with bare ends, and with a handful of chips and a football field this could be built on a Human scale? I sense a Guiness event forming…

Incredible work !! Reminds me my ZX Spectrum days.
You are the guy who will make computers for us after the next world war 🙂
You said you’re not a engineer but Im sure you’re better than one 🙂
Keep the nice work !

[…] This is what Steve meant by “Big Mess of Wires”. Steve recently won an editor’s choice award for the BMOW at Maker Faire. Way to go, Steve! For more information and pictures of the project, visit BMOW. […]

Pavel Precek February 9th, 2011
2:40 am

>You said you’re not a engineer but Im sure you’re better than one
I’m signing under the above sentence.
great job!, impressive!

Anonymous March 15th, 2011
6:58 am

охуенно (awesome on russian).

rocky April 17th, 2011
3:59 pm

well you’ve got yours finished :-), my 2900 slice emulation of (extended 16b) Z80 still remains a box of ic’s and reams of microcode on paper after 15 years …

Well, I just brought BMOW1 out of storage and powered it up for the first time since 2009. It still works great! Maybe I need to build a new clear acrylic case for it now… hiding the wires inside the steel case was a shame.

jak May 10th, 2011
9:34 am

I’ve always wanted to do this!

jeffocojo May 10th, 2011
2:16 pm

this really looks impressive ,just puttin the wraps on the chips must have been something you wish your kids had learned in public school. i used to do 6502 assembly and new all the lab volt rom and its hardware . such wasted time for me ten years to c++. at least you went for it somehow. thanks for the memorys<-pun.

Writing the OS must have been peanuts after achieving this.
Wire-wound, I also haven’t seen that technique in ages.
Must say I feel pretty lazy by buying Atmels stuff, designing hardware and programming them.

Sciphy May 29th, 2011
1:44 am

Hi, great site. I really appreciate you documenting everything. It’s a great location to start from. I might try something similar in a couple of years, can’t quite find enough time now.
Cheers!

yes, great site, and thanks for all this documentation, i m gonna use it

freddy ferrer June 7th, 2011
2:35 pm

que tipos de ttl usaste y si me puedes dar una lista grasias

jowbi.wan August 20th, 2011
8:41 pm

dude. this is the coolest thing i’ve ever seen.

DH August 23rd, 2011
6:15 am

Cool CPU and system. Why are you using a 74×181 ALU, its function can be implemented with shorter propagation times by using simple gates. This has helped inspired me to continue with my CPU design, though it is 16 bit and uses only basic gate ICs, limiting the allowed components to:

@Michael Main:
Systems architecture and and Processor Architecture are Middle School (6th through 8th grade) topics, if you did not master them then you will likely have trouble getting through the first semester of CS studies, the university level courses are covering those sub topics are to be a review (I think that most people test through them so as to save time in getting there degree).

@DH – You could replace the two 74LS181s with a couple of PALs each, or a larger number of basic 7400-series parts, yes. I didn’t realize that at the time, though, and in the end I was limited by available board space and not propagation times. In a different design, replacing the 74LS181 could well make sense.

Systems and processor architecture in middle school? Wow. We didn’t touch any of those topics until my second year of university. Obviously I should have gone to your middle school. 🙂

Post a link to your CPU project if you’ve got one!

DH August 23rd, 2011
7:28 am

If I may recommend: If you ever make a second one, twisting each line with a ground connected line is an effective means to reduce cross talk (make sure to minimize capacitance [by using fine gauge wire]).

I hope that your project gets people to begin to do these types of things more, as it seems that those interested in the core logic are becoming rarer and rarer.

Most of the good designs come from those of us that are self educated, and not Electrical/Electronic Engineers. I am self educated in the world of CS (System Design, Processor Architecture, Software Development, and Electronic Design), Though I DOUBT that my designs will ever be considered great. Your designs on the other hand may become the topics of future CS courses.

DH August 23rd, 2011
7:31 am

Steve:
Now I have to put up a page on my CPU. I hope that no one minds hand drawn schematics. I will give you a link once I get a page up for that (was going to wait for it to be fully functional, but I guess it could be educational).

drago September 3rd, 2011
11:19 am

not, it is intel 8080, made 1980-1985. in will this computer old. Respikt from poland.

Your project inspired me to try to design and build my own computer too, it sure is a lot of work, respect to you for getting yours to work, i hope i can say the same about my project in the future too.

btw, what would you recommend for instruction fetch/decode: in microcode and using hidden general purpose registers, or hardware and using a wide instruction register (7byte wide 1opcode + 2x 24bit address)?

Wow, I have always wanted to do something like this. I have been wanting to make my own computer since I cracked open a windows 95 computer back in early 2000.

Huh CS in middle school, add another person to the list of people wishing that they had gone to that middle school.

Just wondering, is there a parts list for this? Also I have been wondering could this adapted for any past and present processor or just ALU and Z80? I would like to make something like this, just on a bigger board (now where did I put that contact for that company who sells wall sized circuit boards again? just joking!)

Anonymous December 30th, 2013
9:39 pm

Now, if you could do it with 7400 series chips, you should be able to build the same CPU with reed relays!

1_dylan January 25th, 2014
6:17 am

Epic computer! I’m planning on building a 16 bit CPU heavily influenced by your system but I was wandering… What are all the functions the 74LS181 provide?

1_dylan January 25th, 2014
6:20 am

Oh, does it support multiplication because it doesn’t look like it in function tables

Multiplication is done in a software routine, same as it was on the old Apple II and similar 8-bit machines. It’s not fast, but it works. Check out the ‘181 datasheet for a full list of what it does, but it’s basically just addition, subtraction, AND, OR, XOR. A good example of a discrete CPU like BMOW but that’s 16-bit is http://www.timefracture.org/D16.html

A few months ago I designed an 8-bit CPU from scratch. It was very basic. Currently I haven’t been able to build it yet, but it works beautifully on a simulator.

Tonight I found your BMOW project and I must say the mess of wires is truly inspirational! Your Nibbles project has also told me that I can probably build a CPU right here and now: without having to order the parts for my 8-bit design.

Steve March 5th, 2014
2:48 am

I saw in the comments: If you ever make a second one, twisting each line with a ground connected line is an effective means to reduce cross talk (make sure to minimize capacitance [by using fine gauge wire]).
My question is how fine of a wire do I need, since wire wrap is so thin already. I have some transformer wire that is as thin as human hair. Should I wrap it with several coils per unit distance or medium or just a few?
Thanks
Steve

Ra226 January 18th, 2015
7:31 pm

Haha, I hate to tell you, but you are, in fact, an electrical engineer 🙂

rootboy March 15th, 2017
4:54 pm

Very nice!

The last time that I saw something this ambitious was in the 70’s. I had run into an EE who built me a memory board for my Mattel Aquarius computer. While picking it up at his house, he showed me his project (a homegrown CPU like yours).

Where did you get your wirewrap tools? I’m specifically looking for a wirewrap gun.

I actually didn’t use a wirewrap gun – everything was done with a wirewrap hand tool. You can still find some supplies at vendors like Phoenix Enterprises, but they’re definitely getting pretty rare. It’s a dying technique.

rootboy March 15th, 2017
5:35 pm

Sigh… I figured as much. I actually have two of them, but no bits for one of them. And a fairly respectable supply of sockets and wire. 🙂

Just not the patience to strip all of those wires. 🙂

Axel July 19th, 2017
9:19 am

Hello,

How is this possible?
*: A <- A + A; latchNZC; nextop

Latching flags must be done a half clock cycles after the ALU operation? Otherwise there should be timing issues or is there an independent ALU flag register somewhere in GALs?